10 PURPOSES SERVED BY SALIVA. [BOOK II. 



other salts, as for instance, potassium iodide and bromide on peptic 

 digestion. 



in exerting From a consideration of all the facts bearing on the 



their action matter it would appear that in exerting their character- 

 istic actions the various enzymes are in part slowly and 

 gradually destroyed, so that the activity of a given 

 quantity of enzyme cannot be prolonged indefinitely. In most, 

 perhaps in all, cases the accumulation of bodies which result from 

 the ferment action slows and ultimately stops that action long before 

 the enzyme has been exhausted or destroyed, so that, by merely 

 removing the bodies so acting, activity is restored to it. This re- 

 moval can often be effected by the process of dialysis. 



SECT. 1. SALIVA, AND ITS ACTION UPON THE CONSTITUENTS OF 



FOOD. 



INTRODUCTORY SKETCH, CHIEFLY CONCERNING THE 

 SALIVARY GLANDS. 



Purposes The interior of the mouth is continually moistened 



served by by a somewhat viscous, tasteless, watery liquid, the 

 saliva. saliva, a product of the activity of several so-called 



salivary glands ; the presence of this liquid facilitates the movements 

 of the tongue, lips and cheeks in articulation. Though essential to 

 proper articulation, the saliva is, however, to be looked upon as one 

 of the digestive juices, and is poured out in much increased quantities 

 when food is introduced into the mouth. 



It acts as a solvent of many sapid substances introduced into the 

 mouth, and as the vehicle which brings them into contact with the 

 end organs of the nerves of taste ; by moistening the food it ren- 

 ders the essential preliminary act of mastication more easy; it prevents 

 the particles of food from adhering to the interior of the mouth, and 

 thus co-operates with the muscular movements of the lips, tongue, and 

 cheeks in forming the crushed food into a bolus which may readily 

 be propelled through the pharynx and oesophagus; lastly, in man and 

 several other animals it exerts, in virtue of the presence of an enzyme, 

 which used formerly to be termed ptyalin, and which we now usually 

 term the ' diastatic ' or ' amylolytic ' ferment of the saliva, a solvent 

 action upon the starchy constituents of food, and thus initiates the 

 chemical operations to which the food is subjected in its progress 

 through the alimentary canal. The saliva exerts, therefore, two 

 sets of functions, the mechanical and the chemical, of which the 

 first are unquestionably the more important, as is shewn by the fact 

 that in many animals the saliva is free from diastatic enzyme and 

 therefore from any chemical activity whatsoever, or contains it in 

 such small quantities that they cannot be supposed to exert any 

 appreciable action. 



